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A meteorite is a natural object originating in
outer space that survives impact with
the Earth's surface. Most meteorites derive
from small astronomical objects
called meteoroids, but they are also
sometimes produced by impacts of asteroids.
When it enters the atmosphere, impact
pressure causes the body to heat up and emit light, thus
forming a fireball, also known as
a meteor or shooting/falling
star. The term bolide refers to
either an extraterrestrial body that collides with the Earth, or to
an exceptionally bright, fireball-like meteor regardless of whether
it ultimately impacts the surface.

More generally, a meteorite on the surface of any celestial body is
a natural object that has come from elsewhere in space. Meteorites
have been found on the Moon and Mars.

Meteorites that are recovered after being observed as they
transited the atmosphere or impacted the Earth are called
falls. All other meteorites are known as
finds. As of mid-2006, there are approximately
1,050 witnessed falls having
specimens in the world's collections. In contrast, there are over
31,000 well-documented meteorite finds.

Meteorites have traditionally been divided into three broad
categories: stony meteorites are rocks, mainly composed of silicate minerals; iron meteorites are largely composed of
metallic iron-nickel; and, stony-iron meteorites contain large
amounts of both metallic and rocky material. Modern classification
schemes divide meteorites into groups according to their structure,
chemical and isotopic composition and mineralogy. See meteorites classification.

Naming

Meteorites are always named for the place where they were found,
usually a nearby town or geographic feature. In cases where many
meteorites were found in one place, the name may be followed by a
number or letter (e.g., Allan Hills 84001 or Dimmitt (b)).
Some
meteorites have informal nicknames: the Sylacauga meteorite is sometimes called the "Hodges
meteorite" after Ann
Hodges, the woman who was struck by it; the Canyon Diablo
meteorite, which formed Meteor Crater has dozens of these aliases. However, the
single, official name designated by the Meteoritical Society is used by
scientists, catalogers, and most collectors.

Fall phenomena

Most meteoroids disintegrate when entering Earth's atmosphere.
However, an estimated 500 meteorites ranging in size from marbles to basketballs or
larger do reach the surface each year; only 5 or 6 of these are
typically recovered and made known to scientists. Few meteorites
are large enough to create large impact
craters. Instead, they typically arrive at the surface at their
terminal velocity and, at most,
create a small pit. Even so, falling meteorites have reportedly
caused damage to property, livestock and people.

Very large meteoroids may strike the ground with a significant
fraction of their cosmic velocity, leaving behind a hypervelocity impact crater. The kind of
crater will depend on the size, composition, degree of
fragmentation, and incoming angle of the impactor. The force of
such collisions has the potential to cause widespread destruction.
The most frequent hypervelocity cratering events on the Earth are
caused by iron meteoroids, which are most easily able to transit
the atmosphere intact. Examples of craters caused by iron meteoroids
include Barringer
Meteor Crater, Odessa Meteor Crater, Wabar
craters, and Wolfe Creek crater; iron meteorites are found in association with all
of these craters. In contrast, even relatively large stony
or icy bodies like small comets or asteroids, up to millions of tons, are disrupted in
the atmosphere, and do not make impact craters. Although such
disruption events are uncommon, they can cause a considerable
concussion to occur; the famed Tunguska event probably resulted from such an incident.
Very large stony objects, hundreds of meters in diameter or more,
weighing tens-of-millions of tons or more, can
reach the surface and cause large craters, but are very rare. Such
events are generally so energetic that the impactor is completely
destroyed, leaving no meteorites. (The very first example of a stony
meteorite found in association with a large impact crater, the
Morokweng
crater in South Africa, was reported in May
2006.)

Several phenomena are well-documented during witnessed meteorite
falls too small to produce hypervelocity craters. The fireball that
occurs as the meteoroid passes through the atmosphere can appear to
be very bright, rivaling the sun in intensity, although most are
far dimmer and may not even be noticed during daytime. Various
colors have been reported, including yellow, green and red. Flashes
and bursts of light can occur as the object breaks up. Explosions,
detonations, and rumblings are often heard during meteorite falls,
which can be caused by sonic booms as
well as shock waves resulting from major
fragmentation events. These sounds can be heard over wide areas, up
to many thousands of square km. Whistling and hissing sounds are
also sometimes heard, but are poorly understood. Following passage
of the fireball, it is not unusual for a dust trail to linger in
the atmosphere for some time.

As meteoroids are heated during atmospheric entry, their surfaces melt and
experience ablation. They can be sculpted
into various shapes during this process, sometimes resulting in
deep "thumb-print" like indentations on their surfaces called
regmaglypts. If the meteoroid maintains a fixed orientation for
some time, without tumbling, it may develop a conical "nose cone"
or "heat shield" shape. As it decelerates, eventually the molten
surface layer solidifies into a thin
fusion crust, which on most meteorites is black (on some
achondrites, the fusion crust may be very light colored). On stony
meteorites, the heat-affected
zone is at most a few mm deep; in iron meteorites, which are
more thermally conductive, the structure of the metal may be
affected by heat up to 1 cm below the surface. Meteorites are
sometimes reported to be warm to the touch when they land, but they
are never hot. Reports, however, vary greatly, with some meteorites
being reported as "burning hot to the touch" upon landing, and
others forming a frost upon their surface.

Meteoroids that experience disruption in the atmosphere may fall as
meteorite showers, which can range from only a few up to thousands
of separate individuals. The area over which a meteorite shower
falls is known as its strewn field. Strewn fields are commonly
elliptical in shape, with the major axis
parallel to the direction of flight. In most cases, the largest
meteorites in a shower are found farthest down-range in the strewn
field.

Meteorite types

Marília Meteorite, a chondrite H4,
which fell in Marília, São Paulo state, Brazil, on October 5, 1971,
at 5:00p.m.

Most meteorites are stony meteorites, classed as chondrites and achondrites. Only 6% of meteorides are iron meteorites or a blend of rock and metal,
the stony-iron meteorite. Modern
classification of meteorites is complex, the review paper of Krot
et al. (2007) summarizes modern meteorite taxonomy.

About 86% of the meteorites that fall on Earth are chondrites, which are named for the small, round
particles they contain. These particles, or chondrules, are composed mostly of silicate
minerals that appear to have been melted while they were
free-floating objects in space. Certain types of chondrites also
contain small amounts of organic
matter, including amino acids, and
presolar grains. Chondrites are
typically about 4.55 billion years old and are thought to represent
material from the asteroid belt that
never formed into large bodies. Like comets,
chondritic asteroids are some of the oldest and most primitive
materials in the solar system. Chondrites are often considered to
be "the building blocks of the planets".

About 8% of the meteorites that fall on Earth are achondrites (meaning they do not contain
chondrules), some of which are similar to terrestrial mafic
igneous rocks. Most achondrites are
also ancient rocks, and are thought to represent crustal material
of asteroids. One large family of achondrites (the HED meteorites) may have originated on the
asteroid 4 Vesta. Others derive from
different asteroids. Two small groups of achondrites are special,
as they are younger and do not appear to come from the asteroid
belt. One of these groups comes from the Moon, and includes rocks
similar to those brought back to Earth by Apollo and Luna
programs. The other group is almost certainly from Mars and are the only materials from other planets ever
recovered by man.

About 5% of meteorites that fall are iron
meteorites with intergrowths of iron-nickelalloys, such as kamacite and taenite. Most
iron meteorites are thought to come from the core of a number of
asteroids that were once molten. As on Earth, the denser metal
separated from silicate material and sank toward the center of the
asteroid, forming a core. After the asteroid solidified, it broke
up in a collision with another asteroid. Due to the low abundance
of irons in collection areas such as Antarctica, where most of the
meteoric material that has fallen can be recovered, it is possible
that the actual percentage of iron-meteorite falls is lower than
5%.

Stony-iron meteorites constitute the remaining 1%. They are a
mixture of iron-nickel metal and silicate
minerals. One type, called pallasites, is
thought to have originated in the boundary zone above the core
regions where iron meteorites originated. The other major type of
stony-iron meteorites is the mesosiderites.

Tektites (from Greek tektos,
molten) are not themselves meteorites, but are rather natural glass
objects up to a few centimeters in size which were formed—according
to most scientists—by the impacts of large meteorites on Earth's
surface. A few researchers have favored Tektites originating from
the Moon as volcanic ejecta, but this theory
has lost much of its support over the last few decades.

Meteorite recovery

Falls

Most meteorite falls are recovered
on the basis of eye-witness accounts of the fireball or the actual
impact of the object on the ground, or both. Therefore, despite the
fact that meteorites actually fall with virtually equal probability
everywhere on Earth, verified meteorite falls tend to be
concentrated in areas with high human
population densities such as Europe, Japan, and northern India.

A small number of meteorite falls have been observed with automated
cameras and recovered following calculation of the impact point.
The first
of these was the Příbram meteorite, which fell in Czechoslovakia (now the Czech Republic) in 1959. In this case, two cameras used to
photograph meteors captured images of the
fireball. The images were used both to determine the location of
the stones on the ground and, more significantly, to calculate for
the first time an accurate orbit for a recovered meteorite.

Following the Pribram fall, other nations established automated
observing programs aimed at studying infalling meteorites.
One of
these was the Prairie Network, operated by the Smithsonian
Astrophysical Observatory from 1963 to 1975 in the midwestern US. This program
also observed a meteorite fall, the Lost City chondrite,
allowing its recovery and a calculation of its orbit. Another
program in Canada, the Meteorite Observation and Recovery Project,
ran from 1971 to 1985. It too recovered a single meteorite,
Innisfree, in 1977. Finally, observations by the European Fireball Network, a
descendant of the original Czech program that recovered Pribram,
led to the discovery and orbit calculations for the Neuschwanstein meteorite in 2002.

Finds

Until the 20th century, only a few hundred meteorite finds had ever
been discovered. Over 80% of these were iron and stony-iron
meteorites, which are easily distinguished from local rocks. To
this day, few stony meteorites are reported each year that can be
considered to be "accidental" finds. The reason there are now over
30,000 meteorite finds in the world's collections started with the
discovery by Harvey H.Nininger that meteorites are much more
common on the surface of the Earth than was
previously thought.

The Great Plains of the US

Nininger's
strategy was to search for meteorites in the Great Plains of the United States, where the land was largely
cultivated and the soil contained few rocks. Between the
late 1920s and the 1950s, he traveled across the region, educating
local people about what meteorites looked like and what to do if
they thought they had found one, for example, in the course of
clearing a field. The result was the discovery of over 200 new
meteorites, mostly stony types.

In the
late 1960s, Roosevelt County, New Mexico in the Great Plains was found to be a particularly
good place to find meteorites. After the discovery of a few
meteorites in 1967, a public awareness campaign resulted in the
finding of nearly 100 new specimens in the next few years, with
many being found by a single person, Mr. Ivan Wilson. In total,
nearly 140 meteorites were found in the region since 1967. In the
area of the finds, the ground was originally covered by a shallow,
loose soil sitting atop a hardpan layer.
During the dustbowl era, the loose soil was
blown off, leaving any rocks and meteorites that were present
stranded on the exposed surface.

Antarctica

A few
meteorites were found in Antarctica between 1912 and 1964. In 1969, the 10th Japanese Antarctic Research Expedition found nine
meteorites on a blue ice field
near the Yamato Mountains. With
this discovery, came the realization that movement of ice sheets might act to concentrate meteorites in
certain areas. After a dozen other specimens were found in the same
place in 1973, a Japanese expedition was launched in 1974 dedicated
to the search for meteorites. This team recovered nearly 700
meteorites.

Shortly
thereafter, the United States began its own program to search for
Antarctic meteorites, operating along the Transantarctic Mountains on the other side of the continent: the ANtarctic
Search for METeorites (ANSMET)
program. European teams, starting with a consortium called
"EUROMET" in the late 1980s, and continuing with a program by the
Italian Programma Nazionale di Ricerche in Antartide have also
conducted systematic searches for Antarctic meteorites.

The Antarctic Scientific Exploration of China has conducted
successful meteorite searches since 2000. A Korean program
(KOREAMET) was launched in 2007 and has collected a few meteorites.
The combined efforts of all of these expeditions have produced more
than 23,000 classified meteorite specimens since 1974, with
thousands more that have not yet been classified. For more
information see the article by Harvey (2003).

Australia

At about the same time as meteorite concentrations were being
discovered in the cold desert of Antarctica, collectors discovered
that many meteorites could also be found in the hot deserts of Australia. Several dozen
meteorites had already been found in the Nullarbor region of Western and South Australia. Systematic searches between about 1971 and
the present recovered over 500 more, ~300 of which are currently
well characterized. The meteorites can be found in this region
because the land presents a flat, featureless, plain covered by
limestone. In the extremely arid climate,
there has been relatively little weathering or sedimentation on the surface for tens of
thousands of years, allowing meteorites to accumulate without being
buried or destroyed. The dark colored meteorites can then be
recognized among the very different looking limestone pebbles and
rocks.

The Sahara and rising commercialization

In
1986-87, a German team installing a network of seismic stations
while prospecting for oil discovered about 65 meteorites on a flat,
desert plain about 100 km southeast of Dirj (Daraj), Libya. A
few years later, a desert enthusiast saw photographs of meteorites
being recovered by scientists in Antarctica, and thought that he
had seen similar occurrences in northern
Africa. In 1989, he recovered about 100 meteorites from several
distinct locations in Libya and Algeria. Over the next several
years, he and others who followed found at least 400 more
meteorites. The find locations were generally in regions known as
regs or hamadas: flat,
featureless areas covered only by small pebbles and minor amounts
of sand. Dark-colored meteorites can be easily spotted in these
places, where they have also been well-preserved due to the arid
climate, and in the case of the Dal al Gani meteorite field,
favorable geology consisting of basic rocks (clays, dolomites, and limestones)
and lacking erosive quartzsand.

Although meteorites had been sold commercially and collected by
hobbyists for many decades, up to the time of the Saharan finds of
the late 1980s and early 1990s, most meteorites were deposited in
or purchased by museums and similar institutions where they were
exhibited and made available for scientific research. The sudden
availability of large numbers of meteorites that could be found
with relative ease in places that were readily accessible
(especially compared to Antarctica), led to a rapid rise in
commercial collection of meteorites. This process was accelerated
when, in 1997, meteorites coming from both the Moon and Mars were
found in Libya. By the late 1990s, private meteorite-collecting
expeditions had been launched throughout the Sahara. Specimens of
the meteorites recovered in this way are still deposited in
research collections, but most of the material is sold to private
collectors. These expeditions have now brought the total number of
well-described meteorites found in Algeria and Libya to over
2000.

As word
spread in Saharan countries about the growing profitability of the
meteorite trade, meteorite markets came into existence, especially
in Morocco, fed by nomads and local people who combed the
deserts looking for specimens to sell. Many thousands of
meteorites have been distributed in this way, most of which lack
any information about how, when, or where they were discovered.
These are the so-called "Northwest Africa" meteorites.

Arabian Peninsula

In 1999,
meteorite hunters discovered that the desert in southern and
central Oman were also
favorable for the collection of many specimens.The gravel plains in
the Dhofar and Al Wusta regions of Oman, south of the sandy
deserts of the Rub' al
Khali, had yielded about 5,000 meteorites as of
mid-2009. Included among these are a large number of
lunar and Martian meteorites, making Oman a
particularly important area both for scientists and collectors.
Early expeditions to Oman were mainly done by commercial meteorite
dealers, however international teams of Omani and European
scientists have also now collected specimens.

The recovery of meteorites from Oman is currently prohibited by
national law, but a number of international hunters continue to
remove specimens now deemed "national treasures."This new law
provoked a small international
incident, as its implementation actually preceded any public
notification of such a law, resulting in the prolonged imprisonment
of a large group of meteorite hunters primarily from Russia, but
whose party also consisted of members from the U.S. as well as
several other European countries.

The
Black
Stone in the wall of the Kaaba in Mecca is thought
to be a meteorite by some secular historians, but there is little
support for this in the scientific literature

The American Southwest

Beginning in the mid-1990s, amateur meteorite hunters began
scouring the arid areas of the southwestern United States.
To date,
meteorites numbering possibly into the thousands have been
recovered from the Mojave, Sonoran, Great Basin, and
Chihuahuan Deserts, with many being
recovered on dry lake beds.
Significant finds include the Superior Valley 014 Acapulcoite, one
of two of its type found within the United States as well as the
Blue Eagle meteorite, the first Rumuruti-type chondrite yet found
in the Americas. Perhaps the most notable find in recent years has
been the Los Angeles meteorite, a martian meteorite that was
discovered by Robert Verish somewhere in the Mojave desert, only to be recognized years
later in a pile of rocks in his back yard.A number of finds from
the American Southwest have yet to be formally submitted to the
Meteorite Nomenclature Committee, as many
finders think it is unwise to publicly state the coordinates of
their discoveries for fear of confiscation by the federal
government, and of 'poaching' by other hunters at known find
sites.Several of the meteorites found recently are
currently on display in the Griffith Observatory in Los
Angeles.

Meteorites in history

The German physicist, Ernst
Florens Chladni, was the first to publish the then audacious
idea that that meteorites were actually rocks from space. He
published his booklet, "On the Origin of the Pallas Iron and
Others Similar to it, and on Some Associated Natural
Phenomena", in 1794. In this he compiled all available data on
several meteorite finds and falls concluded that they must have
their origins in outer space. The scientific community of the time
responded with resistance and mockery. It took nearly 10 years
before a general acceptance of the origin of meteorites was
achieved through the work of the French scientist Jean-Baptiste Biot and the British
chemist, Edward Howard.

One of the leading theories for the cause of
the Cretaceous–Tertiary
extinction event that included the dinosaurs is a large meteorite impact. The Chicxulub
Crater has been identified as the site of this
impact. There has been a lively scientific debate as to
whether other major extinctions, including the ones at the end of
the Permian and
Triassic periods
might also have been the result of large impact events, but the
evidence is much less compelling than for the end Cretaceous
extinction.

A famous case is the alleged Chinguetti meteorite, a find reputed to
come from a large unconfirmed 'iron mountain' in Africa.

There are several reported instances of falling meteorites having
killed both people and livestock, but a few of these appear more
credible than others. The most infamous reported fatality from a
meteorite impact is that of an Egyptian dog that was killed in
1911, although this report is highly disputed. This particular
meteorite fall was identified in the 1980s as Martian in origin. However, there is substantial
evidence that the meteorite known as Valera hit and killed a cow
upon impact, nearly dividing the animal in two, and similar
unsubstantiated reports of a horse being struck and killed by a
stone of the New Concord fall also abound. Throughout history, many
first and second-hand reports of meteorites falling on and killing
both humans and other animals abound, but none have been well
documented.

Other than the Sylacauga event, the most plausible of these claims
was put forth by a young boy who stated that he had been hit by a
small (~3 gram) stone of the Mbale meteorite
fall from Uganda, and who stood to gain
nothing from this assertion. The stone reportedly fell through a
number of banana leaves before striking the boy on the head,
causing little to no pain, as it was small enough to have been
slowed by both friction with the atmosphere
as well as that with banana leaves, before striking the boy.
Although it is impossible to prove this claim either way, it seems
as though he had little reason to lie about such an event
occurring.

Several persons have since claimed to have been struck by
"meteorites" but no verifiable meteorites have resulted.

Indigenous peoples often prized
iron-nickel meteorites as an easy, if limited, source of iron
metal. For example, the Inuit used chips of the
Cape York
meteorite to form cutting edges for tools and spear
tips.

Some Native Americans treated meteorites as ceremonial objects.
In 1915,
a 135-pound iron meteorite was found in a Sinagua (c.1100-1200 AD) burial cyst near Camp Verde,
Arizona, respectfully wrapped in a feather cloth.A small
pallasite was found in a pottery jar in an old burial found at
Pojoaque
Pueblo, New
Mexico. Nininger reports several other such
instances, in the Southwest US and elsewhere, such as the discovery
of Native American beads of meteoric iron found in Hopewell
burial mounds, and the discovery of the
Winona meteorite in a Native American stone-walled crypt.

In the 1970s a stone meteorite was uncovered during an
archaeological dig at Danebury Iron Age hillfort, Danebury England.
It was found deposited part way down in an Iron Age pit. Since it
must have been deliberately placed there, this could indicate one
of the first (known) human finds of a meteorite in Europe.

The Peruvian meteorite event - On 15 September 2007, a stony meteorite that may
have weighed as much as 4000 kilograms created a crater 13 meters
in diameter near the village of Carancas, Peru.

Apart from meteorites fallen onto the Earth, "Heat Shield Rock" is a meteorite which was
found on Mars, and two tiny fragments of
asteroids were found among the samples collected on the Moon by
Apollo 12 (1969) and Apollo 15 (1971) astronauts.